A mechanism for transmitting a force from the rotation of a motor, such as an AC servomotor and a DC servomotor, to a load via a speed reducer, has been known as a rotary driver for driving a joint of a robot, such as that for an arm, a leg, and a hand. A large torque can be obtained by reducing the number of rotations of the motor by the speed reducer.
Since the rotary driver drives the rotary shaft via the speed reducer, torque is applied to the rotary shaft of the speed reducer. The torque is known to cause a distortion or a slippage of the speed reducer, to thereby affect the accurate operation of a robot. Thus an invention to calculate torque that is applied to the rotary shaft of the speed reducer based on the difference in the rotated angles of a first encoder, which is attached to the rotary shaft of a servomotor, and a second encoder, which is attached to the rotary shaft of the speed reducer, is disclosed (Patent Publication 1).
In a speed reducer that is structured by gears, if a large torque is applied, a phenomenon called tooth-skipping occurs, so that the rotary shaft is rotated to no useful purpose. Thus an error in the number of rotations of the rotary shaft at the input side and that of the rotary shaft at the output side are generated.
An invention is disclosed in which the relative number of the rotations of the rotary shaft at the input side and the rotary shaft at the output side is measured by one encoder (Patent Publication 2). In that invention, if a time derivative of that relative number of rotations exceeds a predetermined rate, it is determined that tooth-skipping has occurred.